1. Field of Invention
The present invention pertains to charge coupled devices (CCDs), and more particularly, to CCDs that have multiple well designs per pixel to increase charge storage capacity.
2. Background of the Invention
CCD imagers are used in a variety of applications, including film digitizers, video cameras, electronic still cameras, FLIR (forward looking infra-red, mostly for military applications), and medical radiography systems. The dynamic range of existing CCD imagers is limited, and inadequate for some applications.
Digitizing radiographic film is a particularly demanding application. The ratio of largest to smallest signal is typically about 3000, corresponding to a film density range of about 3.5 (log 3000.apprxeq.3.5). A typical requirement for signal-to-noise ratio is 15:1, resulting in a dynamic range requirement of about 15 times 3000, approximately 50,000. Some applications require even greater dynamic range. Such a dynamic range is not available with existing CCD imagers.
CCD imagers convert photons (light) to electrons (or holes), and accumulate the electrons in wells. The maximum signal in such a system is limited by the number of electrons that the wells can hold before overflowing. The capacity of the wells can be increased, but generally this also increases the electronic noise. Thus the dynamic range is limited, typically far below 50,000.
U.S. Pat. No. 5,221,848 discusses constructing two CCD sensor arrays, one for weak signals and one for strong signals, to increase the dynamic range. U.S. Pat. Nos. 5,268,567; 5,283,426; 5,055,667; and 4,873,561 discuss modifying the CCD sensor and/or control electronics to produce a non-linear response, which increases the largest usable signal, thereby increasing the dynamic range.
In U.S. Pat. No. 5,221,848, issued to Milch (hereinafter referred to as Milch), Milch provides wide dynamic range by providing separate sensors for low and high signals. With this approach, the low signal sensor may see a different image than the high signal sensor, because they are at different locations, may be exposed at different times or for different exposure periods, and may be exposed through different filters. Another problem with the teachings of the disclosure of Milch is that it discards some of the available signal, e.g. by reducing the exposure time or applying a neutral density filter to the light. By discarding signal, this worsens the signal/noise ratio.
U.S. Pat. Nos. 5,268,567; 5,283,426; 5,055,667; and 4,873,561 all discard signal, by various means, and thus worsen the signal/noise ratio, as compared with the tandem CCD design. The tandem CCD design also has lower noise (and thus better signal/noise ratio) than U.S. Pat. No. 4,873,561, because the tandem design separates the small and large wells by a barrier, so that dark current noise generated in the large wells does not leak into the small wells; while in the U.S. Pat. No. 4,873,561 design dark current noise generated in the large wells flows easily into the small wells, thereby corrupting even the small signals with large amounts of noise.
A typical prior art, linear CCD imager is shown in FIG. 1. The photodiodes convert incident photons to electrons during the exposure time interval. After the exposure, the Gate is lowered, allowing the electrons to move into the adjacent CCD wells. Then the chain of CCD wells is clocked, bucket brigade style, moving the charge in each CCD well to the next CCD well. The end CCD well is clocked into an amplifier, then into an analog to digital converter, producing a digital signal. Each pixel position comprises a photodiode and a CCD well. The dashed line encloses the portion that is typically on a single integrated circuit. This example shows a 4 pixel CCD imager. Typical linear CCD imagers contain hundreds or thousands of pixels.
During exposure to light, the photons generate electron/hole pairs in the photodiode. In most devices the electrons are used, and the holes are discarded. After the exposure is complete, the gate is lowered, allowing the electrons to flow from the photodiode into the CCD well. If the light signal is too strong, the capacity of the CCD well is exceeded, i.e. there are more electrons than the well can hold. In some devices, to prevent the excess electrons from flowing to adjacent pixels (a phenomenon called blooming), an overflow gate and overflow drain are provided. Properly adjusting the voltage on the overflow gate limits the number of electrons which can accumulate in the photodiode. The excess electrons flow over the overflow gate, into the overflow drain, where they are removed. FIG. 2 shows the energy diagram for a single pixel.
Referring to FIG. 2, which is an energy diagram of one pixel of the typical prior art CCD imager shown in FIG. 1, the photons create electrons in the photodiode. The gate is then lowered, and the electrons drain into the CCD well. The height of the overflow gate can be set to limit the total number of electrons that accumulate in the photodiode, by allowing excess electrons to pass to the overflow drain. The overflow gate and overflow drain also are sometimes used to provide electronic shutter control. In such cases, the overflow gate is fully lowered for a period of time, then raised. Electrons accumulate in the photodiode only while the overflow gate is raised. Thus, extending the period when the gate is lowered decreases the time interval when electrons accumulate, thereby decreasing the exposure.
After the CCD wells receive charge from the photodiodes, the row of CCD wells are clocked out to the amplifier, bucket brigade style. The charge in each CCD well is passed to the adjacent CCD well. The charge from the end CCD well is passed to the amplifier. This charge is amplified and passed to an analog-to-digital converter (A/D), where the analog signal is converted to a digital number. This clocking is repeated until every CCD well, one at a time, is passed to the amplifier and A/D, resulting in a series of digital numbers, one number corresponding to the light signal collected at each of the photodiodes.
In most designs, the maximum signal that can be sensed is governed by the electron capacity of the CCD wells. Typical devices have CCD well capacities of between 50,000 and 1,000,000 electrons. The CCD well capacity can be increased, but there is a corresponding increase in electronic noise, so that the signal to noise ratio is limited.